AIMS: Amgevita is a licensed biosimilar to adalimumab, having demonstrated high pharmacokinetic and clinical similarity to Humira. Switching to a lower-cost medicine may elicit a nocebo effect, whereby expectations of poorer efficacy impact outcomes despite pharmacological similarity. This prospective cohort study examined clinical and economic outcomes and associated psychosocial variables in a group of patients undergoing a nonmedical switch to biosimilar adalimumab. METHODS: Patients with inflammatory bowel disease (IBD) were followed before and after switching from Humira to Amgevita. Objective disease activity was assessed pre- and post-switch using the Harvey-Bradshaw Index (Crohn's disease) or partial Mayo score (ulcerative colitis), faecal calprotectin and C-reactive protein. Subjective symptom burden was measured using the IBD Control Questionnaire (IBDCQ). Pre-switch, health anxiety was measured using the Health Anxiety Index (HAI). RESULTS: In total, 64 patients aged 18-67 were enrolled. IBDCQ scores marginally improved post-switch (13.33 vs, 12.49, P = .043), with no significant changes in objective disease activity scores, faecal calprotectin or C-reactive protein. Sixteen patients reported 17 new adverse events within 4 weeks. Logistic regression revealed a significant relationship between HAI scores and adverse events (P = .0079); each unit increase in HAI score increased the odds of reporting an adverse event by 21%. Drug cost savings for the 64 patients over 8 weeks totalled euro143 958. CONCLUSION: Switching to biosimilar adalimumab did not affect disease control or quality of life. 25% of patients developed new side effects, particularly those with high levels of health anxiety. Significant cost savings were achieved.